1. Field of the Invention
This invention relates to a process for recovering oil from a subterranean formation by injecting a microemulsion into the formation to displace the oil to a production well. The microemulsion is specially formulated to enable the compatible incorporation of a mobility-control polymer.
2. Description of the Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original oil in place in a reservoir is expelled by natural mechanisms. It is also well known that conventional methods of supplementing natural recovery are relatively inefficient. Typically, a reservoir may retain half its original oil even after the application of currently available methods of secondary recovery. Accordingly, there is a continuing need for improved recovery methods which will substantially increase the ultimate yield of petroleum from subterranean reservoirs.
Waterflooding is by far the most economical and widely practiced of secondary recovery methods. In such a process, water is injected through an input well to drive oil from the formation and to an offset producing well. Much of the current work in secondary recovery technology has been directed toward improving the efficiency of waterflooding processes.
Surface active agents or surfactants are one class of materials which have been proposed for improving the efficiency of waterflooding processes. Much of the oil that is retained in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir. It has been suggested that, because the normal interfacial tension between the reservoir oil and water is so high, these discrete droplets are unable to sufficiently deform to pass through narrow constrictions in the pore channels. When surface-active agents are added to the flooding water, they lower the interfacial tension between the water, they lower the interfacial tension between the water and the reservoir oil and permit the oil droplets to deform and flow with the flood water. It is generally conceded that the interfacial tension between the flood water and the reservoir oil must be reduced to less than 0.1 dynes/cm. for effective recovery.
While conventional surfactant waterflooding may be effective in obtaining additional oil from subterranean oil reservoirs, it has a number of shortcomings which detract seriously from its value. Foremost among these shortcomings is the tendency of surfactant flood water to finger through the reservoir and to bypass substantial portions of oil. This fingering tendency of a surfactant waterflood is usually explained by the fact that the surfactant flood water has the ability to move through the reservoir at a much faster rate than the oil which it is displacing. The fingering and bypassing tendencies of the surfactant flood water is due in part to its relatively low viscosity.
The mobility ratio of a flooding system is a mathematical relationship that has developed to help explain the behavior of fluids flowing through porous media such as oil reservoirs. When the mobility ratio equation is applied to a "flooding" type operation within a reservoir, it reads as follows: ##EQU1## where: M is the mobility ratio
.lambda..sub.o is the mobility of the oil in the reservoir PA1 .lambda..sub.w is the mobility of the driving fluid in the reservoir PA1 .mu..sub.o is the viscosity of the driven oil PA1 .mu..sub.w is the viscosity of the driving fluid PA1 k.sub.w is the relative permeability of the reservoir to the driving fluid in the presence of residual oil PA1 k.sub.o is the relative permeability of the reservoir to the oil in the presence of residual driving fluid.
This equation is perhaps best explained by stating that when the mobility ratio of the driving fluid to oil is equal to one, the oil and the driving fluid move through the reservoir with equal ease. When the mobility ratio is greater than one, there is a tendency for driving fluid to bypass the oil and finger to the producing well.
It should be noted that crude oils vary greatly in viscosity. Some have viscosities as low as 1 or 2 centipoise and some range up to 1,000 centipoise or greater. Most reservoir oils have a viscosity of up to 10 centipoise at reservoir temperature and pressure. If a surfactant waterflood is conducted using an injection composition where viscosity is approximately 1 centipoise and the oil to be displaced has a viscosity of 10 centipoise, it can be seen from the mobility ratio equation that there will be a tendency for the driving fluid to finger through the reservoir oil. It has in fact been noted that surfactant waterflooding generally performs less satisfactorily with viscous crude oils than with relatively non-viscous oils.
One method for improving conventional surfactant waterflooding techniques is to use microemulsions. Microemulsions are stable, transparent or translucent mixtures of a liquid hydrocarbon, water and a surfactant. Optionally, a co-solvent such as alcohol and electrolytes may be present in the mixture. Generally, microemulsions may be oil-external, water-external or microemulsions wherein no external phase can be identified. In practice, a microemulsion is injected into the formation and displaced through the formation by means of a driving fluid such as thickened water. While the use of microemulsions tends to improve the mobility ratio between it and the reservoir fluids, the problem of fingering and bypassing still occurs.
Several approaches have been suggested to date for improving the mechanics of microemulsion flooding specifically with the view of reducing the degree of fingering and bypassing. The most obvious approach is to increase the viscosity of the microemulsion relative to the oil. A wide variety of materials and formulations have been suggested for accomplishing this purpose. For example, U.S. Pat. No. 3,719,606 to Froning, et al discloses a specially formulated microemulsion containing a sulfonate surfactant, an alcohol co-surfactant, a polysaccharide thickener, an aqueous brine and a hydrocarbon oil. By utilizing closely defined amounts of these materials to prepare the microemulsion substantial improvement in mobility control is claimed. Another example is U.S. Pat. No. 3,827,496 to Schroeder which discloses another special formulation including the incorporation of a viscosity increasing agent within the microemulsion in order to increase microemulsion viscosity. U.S. Pat. No. 3,981,361 to Healy suggests a method for designing a microemulsion system which includes a thickener. These patents are representative of the approaches used and recognized advantages in controlling the mobility of a microemulsion for injection into a formation. Throughout this disclosure, the expressions "polymer" and "thickener" will be used interchangeably to indicate a viscosifying agent which can be added to a liquid to increase the viscosity of the liquid.
Unfortunately, it has been discovered that it is practically impossible to physically dissolve sufficient quantities of polymer into a microemulsion at or near optimal salinity; this problem is particularly acute when this salinity is high, typical of most reservoir brines. The optimal salinity for a given surfactant approximately equals that concentration of inorganic salts in a microemulsion at which a low interfacial tension exists for both a microemulsion-oil interface and a microemulsion-water interface; the expression is defined more precisely in the Definitions section. Many reservoirs contain brines of high salinity, e.g. NaCl concentrations of about 50 g/l or greater up to saturation, CaCl.sub.2 concentrations of about 5 g/l or greater up to saturation, MgCl.sub.2 concentrations of 5 g/l or greater up to saturation, along with trace amounts of other salts. These high salinities present a particularly severe problem since it is known that the oil recovery tends to be optimized in those cases where the primary surfactant is chosen such that its optimal salinity is close to the salinity of the reservoir in question and where the salinity of the microemulsion is similar to that of reservoir brine. Also, it is highly desirable to inject single-phase microemulsion compositions and, after injection, for such microemulsions to remain as a single phase for as long as possible during movement through the formation.
Various approaches have been taken by others to compatibly incorporate polymer and surfactant into a microemulsion. One approach taken is to add large amounts of costly chemicals to help solubilize the polymer. The economics of this approach often are such that this is not a feasible solution to the problem. Another approach is to utilize a microemulsion system which contains an aqueous component having a reduced salinity with respect to reservoir salinity (i.e. not at optimal salinity). This creates even more problems in that reducing the salinity leads to an increase in oil/microemulsion interfacial tension and hence reduced oil recovery. Finally, even where polymer can be incorporated, phase separation can rapidly occur as the microemulsion flows through the formation resulting in inefficient oil recovery.
It is clear, therefore, that serious and fundamental problems exist with incorporating a viscosity increasing agent into a microemulsion at or near a surfactant's optimal salinity. The crucial and heretofore unsolved problem is to compatibly dissolve sufficient amounts of the thickening agent in the presence of the surfactant. Prior art processes generally require trade-offs between mobility control and interfacial tension reduction. Many of the prior art processes wherein a thickener is suggested to be incorporated into the microemulsion require the use of high amounts of oil or uneconomical amounts of expensive co-solvents and the like to solubilize both surfactant and thickener. In reality, a microemulsion flooding process will not be taken to the field unless the microemulsion uses low amounts of oil and chemicals. In this situation, the approaches of the prior art become inoperable quite simply because sufficient amounts of viscosity increasing agents cannot be dissolved in the microemulsion. For all practical purposes, without the incorporation of such viscosifiers, economic recovery of oil by microemulsion flooding is impossible due to adverse mobilities; similarly, mobility control without sufficient reduction in interfacial tensions is unacceptable.